1. Field of the Invention
The present invention relates to a GaN-based III-V group compound semiconductor device and a p-type electrode for the same.
2. Description of the Related Art
To embody an optical device, such as a light emitting diode (LED) or a laser diode (LD), using GaN-based compound semiconductor, it is very important to form a high-quality ohmic contact between a semiconductor layer and an electrode. An ohmic contact layer for a p-type GaN semiconductor layer may be a Ni-based metal thin layer, i.e., a transparent metal thin layer formed of Ni and Au (refer to U.S. Pat. Nos. 5,877,558 and 6,008,539).
As is known, when a Ni-based thin layer is annealed in an O2 atmosphere, an ohmic contact having a low specific contact resistance of about 10−3 to 10−4 Ωcm2 is formed. When the N-based thin layer is annealed in the O2 atmosphere at a temperature of about 500 to 600° C., due to the low specific contact resistance, nickel oxide (NiO), which is a p-type semiconductor oxide, is formed between an island-shaped Au layers and on the Au layers at an interface between a GaN layer and a Ni layer, thereby reducing a Schottky barrier height (HBT). Thus, as the HBT is reduced, majority carriers, i.e., holes are easily supplied near to the surface of the GaN layer so that effective carrier concentration can increase near the surface of the GaN layer. Meanwhile, when a Ni/Au layer is brought into contact with a p-type GaN semiconductor layer and is annealed, an intermetal complex between Mg and H is removed. This leads to reactivation, which increases a Mg-waveguident concentration in the surface of the p-type GaN semiconductor layer. As a result, the effective carrier concentration is raised to 1019/cm3 or higher to cause a tunneling phenomenon between the p-type GaN semiconductor layer and an electrode layer (i.e., the NiO layer), thereby exhibiting an ohmic conducting characteristic. However, since the Ni/Au thin layer is not appropriate for an LD that requires a lower ohmic contact resistance, a new ohmic contact material having a sufficiently low specific contact resistance is needed.